1. Field of the Invention
The present invention relates to a base station of a mobile communication system, and more particularly to a base station system supporting multi-sector/multi-frequency assignment.
2. Description of the Related Art
In general, a base station of a mobile communication system such as a Universal Mobile Telecommunication System (UMTS) has a construction capable of supporting multi-sector and multi-frequency assignment (hereinafter, referred to as FA), so as to increase a user capacity and efficiently utilize the frequency.
FIG. 1 is a block diagram illustrating a construction of a conventional base station system of a mobile communication system, which can support multi-sector/multi-FA and provide an antenna diversity effect. In this case, it is assumed that the base station system supports six sectors/four FAs, although FIG. 1 shows only one sector and four FAs supported by the base station system, for convenience of description.
Referring to FIG. 1, a signal received by a first receiving antenna Rx Ant. 1 is band pass-filtered by a band pass filter (BPF) 11. A low noise amplifier (LNA) 13 amplifies the signal which has been band pass-filtered by the band pass filter 11. A divider 15 divides the signal amplified by the low noise amplifier 13 into signals for each of the four FAs. That is, the signals divided by the divider 15 are applied to an up/down-converter board 21 for FA1, an up/down-converter board 22 for FA2, an up/down-converter board 23 for FA3, and an up/down-converter board 24 for FA4, respectively.
A signal received by a second receiving antenna Rx Ant. 2 is band pass-filtered by a band pass filter 12. A low noise amplifier 14 amplifies the signal which has been band pass-filtered by the band pass filter 12. A divider 16 divides the signal amplified by the low noise amplifier 14 into signals for each of the four FAs. That is, the signals divided by the divider 16 are applied to an up/down-converter board 21 for FA1, an up/down-converter board 22 for FA2, an up/down-converter board 23 for FA3, and an up/down-converter board 24 for FA4, respectively.
The four up/down-converter boards 21 to 24 for the four FAs constitute an up/down-converter 20. Each board includes first down-converters 21a, 22a, 23a, and 24a for downwardly converting frequencies of signals processed after being received through the first receiving antenna Rx Ant. 1, second down-converters 21b, 22b, 23b, and 24b for downwardly converting frequencies of signals processed after being received through the second receiving antenna Rx Ant. 2, and up-converters 21c, 22c, 23c, and 24c for upwardly converting frequencies of signals to be transmitted.
A combiner 31 receives and combines the four signals with the upwardly-converted frequencies from the four up/down-converter boards 21 to 24 of the up/down-converter 20, and outputs the combined signal. A divider 32 divides again the signal combined by the combiner 31 into four signals, which can be inputted to four power amplifiers (PA) 33 to 36. The four power amplifiers 33 to 36 correspond to the four FAs, respectively. A combiner 37 combines the four signals from the power amplifiers 33 to 36 and outputs a combined signal. The band pass filter 38 band pass-filters the combined signal and outputs it through a transmission antenna Tx Ant.
In the base station system as illustrated in FIG. 1 (which is called “Node B” in the UMTS), the power amplifiers 33 to 36 and the four up/down-converter boards 21 to 24 are elements which require automatic switching when the boards are abnormal. The base station system having the construction described above employs a hard-redundancy mode, utilizing extra boards when it has a small number of sectors and FAs. When the base station has an increased number of sectors and FAs, only the power amplifier employs a soft-redundancy mode utilizing load sharing, due to the problem of the number of the boards. In this case, when one of the four up/down-converter boards 21 to 24 is out of order, an FA of a sector corresponding to the out-of-order board cannot be used. That is, in the conventional base station system, one FA becomes useless when a board is out of order, since transmitting and receiving paths according to the FAs are formed in one up/down converter.
In the conventional base station system as described above, although each board independently operates in order to support the multi-sector/multi-FA, when one of the boards is out of order, the call is cut off, thereby having a detrimental effect on the entire capacity of the system.